While true, you would have to ignore all other indicators for quite an extensive period of time. Like excessive urination and hypersensitivity being obvious ones. Not impossible but I have the strong sense there is more to this story than reported in the FDA disclosure.
It's not that surprising, a lot of people (especially doctors) will dismiss symptoms if "objective" tests show normal levels
Not true, the effect of high blood levels can be very unpredictable. Especially above a certain number - and largely depending on the basal insulin strategy of the patient, for instance long acting shots versus constant micro doses of short acting via pumps. In the latter case, an untreated high blood sugar could escalate in a matter of hours to a fatal level.